Modular dielectric barrier discharge device for pollution abatement

ABSTRACT

A device ( 10 ) for purifying a gas stream ( 30 ) made up of a plurality of DBD cells ( 12   a,    12   b,    12   c ) in series and, for each of the DBD cells ( 12   a,    12   b,    12   c ), a power supply ( 24   a,    24   b,    24   e ) for providing alternating current to each DBD cell ( 12   a,    12   b,    12   e ).

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to pollution abatement and, moreparticularly, to a device for destroying gaseous pollutants by passingthe pollutants through a plasma and possibly additional gas.

Gases that are hazardous or otherwise undesirable are produced by manycommercial and industrial processes. Notable examples include oxides ofnitrogen and sulfur, emitted, for example, from internal combustionengines and from power plants; chemical and biological agents such assarin and tabun; and fluorine-containing greenhouse gases(perfluorocarbons) such as CF₄, CHF₃, C₂HF₅, C₂H₂F₄ and SF₆ that areused in the fabrication of semiconductor devices. There are threegeneral method to control emissions of these gases:

1. Control the processes that generate or use such gases to minimizetheir production or use.

2. In the case of gases deliberately introduced to industrial processessuch as semiconductor device fabrication, collect and recycle theemitted gases.

3. Convert the gases to environmentally safer compounds.

The present invention addresses the third general method. Traditionally,the semiconductor industry has incinerated effluent gases. The burnersused tend to be large, inefficient and expensive. Recently, it has beenproposed to use plasmas, such as are used for generating ozone fromoxygen, to destroy unwanted gaseous species. The high energy electronsof a plasma deliver their energy efficiently to atoms and moleculeswithout heating the device which creates the plasma. The modification ofthe gas molecules is done by direct interaction with the electronsthrough electron attachment, dissociation or ionization, or throughinteraction with free radicals generated by the electrons.

There are two types of plasmas that may be used for pollution abatement:thermal plasmas and non-thermal plasmas. A thermal plasma is one that isin thermal equilibrium. Such plasmas may be generated by, for example,continuous RF or microwave energy. The particle energy in the plasma isa function of the plasma temperature, on the order of kT, where k isBoltzmann's constant and T is the plasma temperature. For typicalthermal plasmas, the particle energy is on the order of electron volts.Non-thermal plasmas generate much higher electron energies, andtherefore are characterized by more efficient energy transfer thanthermal plasmas. The disadvantage of non-thermal plasmas is that theyare more difficult to control and to keep uniform than are thermalplasmas.

Two types of non-thermal plasmas have been considered for pollutionabatement: pulsed corona discharge and dielectric barrier discharge(DBD). In pulsed corona discharge, the plasma is generated between twoelectrodes by a pulse of high voltage across the electrodes, whichcreates a discharge in the gas between the electrodes. To prevent thecreation of a single arc discharge which would carry the entire currentand create a non-uniform plasma, the voltage pulse is kept short, on theorder of tens of nanoseconds, and is repeated at a rate on the order ofhundreds of times per second. The plasma discharge channels thus createddo not have enough time to turn into an arc, so many discharge channelsare created during the short lifetime of the pulse. Nevertheless, it isdifficult to create a very uniform corona discharge. A representativeU.S. patent describing a pulsed corona reactor is U.S. Pat. No.5,490,973, to Grothaus et al.

In a DBD device, one or both of the electrodes are covered with aninsulator so that the energy for the discharge is suppliedcapacitatively through the insulator. This limits the amount of energythat each discharge channel can receive. It therefore is possible togenerate more channels and obtain a more uniform discharge.

SUMMARY OF THE INVENTION

According to the present invention there is provided a device forpurifying a gas stream, including: (a) a plurality of DBD cells inseries; (b) for each of the DBD cells, a power supply for providingalternating current to the each DBD cell.

According to the present invention there is provided a method forpurifying a gas stream, including the steps of: (a) providing aplurality of DBD cells in series; and (b) causing the gas stream to flowthrough said DBD cells.

The basic structure of the present invention is a plurality of DBD cellsin series. By “series” is meant, not that the cells are electrically inseries, for indeed each cell is part of an independent electricalcircuit, but that the cells are arranged geometrically so that the gasstream to be purified passes sequentially from one cell to the next.Each cell is provided with its own independent high frequency powersupply. In conformity with common usage, these power supplies arereferred to herein as supplying “alternating current” to the DBD cells,although the parameter of the power supplies that actually is controlledis the voltage, with the supplied currents then depending on theimpedances of the DBD cells according to Ohm's law. Preferably, thepower supplies are switching mode resonant power supplies.

The use of several small DBD cells instead of one large DBD cell has thefollowing advantages:

1. The smaller capacitance of a small cell makes it easier to drive athigh frequencies. At higher frequencies, more discharge channels arecreated, so the plasma is more uniform. The smaller power supplies usedwith the smaller cells are simpler and more efficient than the largepower supply that would be needed for a single large cell.

2. A plurality of cells is easier to control than a single cell. It iseasier and more efficient to control the concentrations of chemicalspecies inside a plurality of small cells than inside a single largecell. According to the present invention, sensors are provided tomeasure the concentrations of gaseous species emerging from each celland plasma conditions inside each cell. Power supply parameters such asfrequency and voltage are adjusted adaptively, in accordance with theresults of the measurements, to enhance the destruction of the unwantedspecies.

3. A reactor made of a plurality of cells is modular. If one cell mustbe taken off line for maintenance, the reactor can continue to function.

The scope of the present invention also includes the injection of anadditive gas, such as nitrogen or oxygen, into the gas stream, at theinlet to one or more of the cells, to enhance the destruction of theunwanted gaseous species and their conversion to safe gases. As in thecase of the power supply parameters, the rate of injection of theadditive gas is controlled in accordance with the measuredconcentrations and plasma conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of a basic device of the presentinvention;

FIGS. 2A through 2F are cross sections of alternative constructions of aDBD cell;

FIG. 3 is a schematic axial cross section of another DBD cell;

FIG. 4 is a schematic diagram of a power supply;

FIG. 5 is a schematic illustration of a preferred device of the presentinvention;

FIG. 6 is an axial cross section of an improved embodiment of the DBDcell of FIG. 2E.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is of a modular DBD reactor which can be used todestroy pollutant species in a gas stream, converting the pollutantspecies to environment-friendly gases. Specifically, the presentinvention can be controlled adaptively to optimize the destruction ofthe unwanted species.

The principles and operation of a modular DBD reactor according to thepresent invention may be better understood with reference to thedrawings and the accompanying description.

Referring now to the drawings, FIG. 1 is a schematic illustration of abasic device 10 of the present invention. Device 10 includes three DBDcells 12 a, 12 b and 12 c. Each DBD cell includes two electrodes: cell12 a includes electrodes 14 a and 16 a, cell 12 b includes electrodes 14b and 16 b, and cell 12 c includes electrodes 14 c and 16 c. Electrode14 a is covered by a dielectric layer 18 a. Electrode 14 b is covered bya dielectric layer 18 b. Electrode 14 c is covered by a dielectric layer18 c. Electrode 16 a is covered by a dielectric layer 20 a. Electrode 16b is covered by a dielectric layer 20 b. Electrode 16 c is covered by adielectric layer 20 c. Dielectric layers 18 a and 20 a define betweenthem a gap 22 a. Dielectric layers 18 b and 20 b define between them agap 22 b. Dielectric layers 18 c and 20 c define between them a gap 22c. Electrodes 16 a, 16 b and 16 c are grounded. Electrodes 14 a, 14 band 14 c are connected to high frequency power supplies 24 a, 24 b and24 c respectively.

Each DBD cell has an input end, into which a gas stream 30 to bepurified enters, and an output end, from which gas stream 30 exits aftertreatment in that cell: cell 12 a has an input end 26 a and an outputend 28 a, cell 12 b has an input end 26 b and an output end 28 b, andcell 12 c has an input end 26 c and an output end 28 c. The cells arearranged in series, so that gas stream 30, after exiting cell 12 a viaoutput end 28 a, immediately enters cell 12 b via input end 26 b, andafter exiting cell 12 b via output end 28 b, immediately enters cell 12c via input end 26 c.

Electrodes 14 a, 14 b, 14 c, 16 a, 16 b, and 16 c are made of anelectrically conductive material, preferably a metal, most preferablycopper, aluminum or stainless steel. Dielectric layers 18 a, 18 b, 18 c,20 a, 20 b and 20 c are made of an electrical insulator, preferably aceramic such as alumina or quartz. For simplicity, only three DBD cellsare shown in FIG. 1. Typically, device 10 includes 10 cells, but thescope of the present invention includes any convenient number of cellsin device 10 greater than or equal to 2. If more than 5 cells are used,one cell may be taken off line for maintenance without disabling theentire device.

FIGS. 2A, 2B, 2C, 2D, 2E and 2F show alternative constructions of DBDcells. FIG. 2A is an axial cross section of a cell 12 d in which onlyone electrode 14 d is covered by a dielectric layer 18 d; electrode 16 dis bare. Electrode 16 d and dielectric layer 18 d define between them agap 22 d through which gas stream 30 flows. FIGS. 2B, 2C, 2D, 2E and 2Fare transverse cross sections of cylindrical DBD cells. FIG. 2B shows acell 12 e that includes two electrodes 14 e and 16 e, in the form ofcylindrical sections, on opposite sides of a dielectric tube 18 e. In adevice 10 including cells such as cell 12 e, gas stream 30 flows throughinterior 22 e of dielectric tube 18 e. FIG. 2C shows a cell 12 f inwhich one electrode is an electrically conductive cylinder 14 f and theother electrode is an electrically conductive wire 16 f concentric withcylinder 14 f. The inner surface of cylinder 14 f is coated with acylindrical dielectric layer 18 f. In a device 10 including cells suchas cell 12 f, gas stream 30 flows through interior 22 f of cylinder 18f. FIG. 2D shows a cell 12 g in which the electrodes are concentric,electrically conductive cylinders 14 g and 16 g. The inner surface ofcylinder 14 g is coated with a cylindrical dielectric layer 18 g. Theouter surface of cylinder 16 g is coated with a cylindrical dielectriclayer 20 g. In a device 10 including cells such as cell 12 g, gas stream30 flows through annulus 22 g defined by dielectric cylinders 18 g and20 g. FIG. 2E shows a cell 12 h in which the electrodes are concentric,electrically conductive cylinders 14 h and 16 h, cylinder 16 h beingsolid rather than hollow. Cylinder 14 h is bare. The surface of cylinder16 h is coated with a cylindrical dielectric layer 20 h. In a device 10including cells such as cell 12 h, gas stream 30 flows through interior22 h of cylinder 14 h. FIG. 2F shows a cell 12 i in which there arethree concentric electrodes: hollow, electrically conductive cylinders14 i and 15, and solid, electrically conductive cylinder 16 i. The innersurface of cylinder 14 i is coated with a cylindrical dielectric layer18 i. The surface of cylinder 16 i is coated with a cylindricaldielectric layer 20 i. In a device 10 including cells such as cell 12 i,gas stream 30 flows through both an annulus 23 defined by cylinders 15and 20 i and an annulus 23’ defined by cylinders 18 i and 15. In theoperation of a cell such as cell 12 i, cylinder 15 is connected to apower supply such as power supply 24 a, 24 b or 24 c, and both cylinders14 i and 16 i are grounded.

FIG. 6 is an axial cross section of an improved embodiment 112 of cell12 h. Cell 112 includes a cylindrical outer electrode 114 and an innerelectrode 116 having an axially varying transverse width w. Inparticular, w varies sinusoidally with a decreasing amplitude from inputend 126 to output end 128. Inner electrode 116 is coated with adielectric layer 120 whose transverse width also varies axially. Innersurface 117 of outer electrode 114 is coated with a layer 118 of acatalyst such as black platinum or titanium for catalyzing thedestruction of the pollutant species in gas stream 30. Cell 112 alsohas, at output end 128, an exit aperture 130 that limits the velocity ofgas stream 30, thereby increasing the pressure of the gas in interior122 of cell 112. Alternatively, catalyst layer 118 is on inner electrode116 and dielectric layer 120 is on inner surface 117 of outer electrode114.

Typically, the lengths of DBD cells of the present invention are on theorder of several centimeters, as are the diameters of cylindrical DBDcells and the widths of planar cells. The thicknesses of the dielectriclayers and the widths of the gaps between dielectric layers, or betweena dielectric layer and an opposite bare electrode, typically are on theorder of several millimeters.

FIG. 3 is a schematic axial cross section of a DBD cell 12 j that isgeometrically similar to cells 12 a, 12 b and 12 c, having twoelectrodes 14 j and 16 j whose facing surfaces are coated withdielectric layers 18 j and 20 j, dielectric layers 18 j and 20 jdefining between them a gap 22 j. Cell 12 j is provided with a mechanismfor changing width 21 of gap 22 j. Specifically, cell 12 j is mountedwithin an insulating housing that consists of an upper part 32 rigidlyattached to electrode 14 j and a lower part 34 rigidly attached toelectrode 16 j. Parts 32 and 34 have matching threaded holes throughwhich are inserted threaded rods 36. Threaded rods 36 are extensions ofthe shafts of stepping motors 38. Stepping motors 38 are activated asdescribed below to rotate rods 36 to change width 23 during theoperation of a device 10 that includes a cell such as cell 12 j. Themechanism illustrated in FIG. 3 is only illustrative: the scope of thepresent invention includes all suitable mechanisms for adjusting theinterior geometries of the DBD cells.

Preferably, power supplies 24 a, 24 b and 24 c are switching moderesonant power supplies, which are simple, efficient and inexpensive.FIG. 4 is a schematic diagram of a representative such power supply 24.Power supply 24 includes a DC power source 40 in series with a switch44, a variable inductance 46, and the primary winding of a transformer48; and in parallel with a capacitor 42. The secondary winding oftransformer 48 is shown supplying the output AC current of power supply24 to a DBD cell 12 represented by an equivalent circuit that includes acapacitance 50 in parallel with a resistance 52. Power source 40supplies a DC voltage on the order of several tens to hundreds of volts.Capacitor 42 is of low equivalent series resistance, to enable high peakcurrents through the primary coil of transformer 48. Transformer 48isolates power supply 24 from cell 12 and matches the load voltage andimpedance of cell 12. Typically, the peak voltage supplied by thesecondary winding of transformer 48 to cell 12 is on the order of about300 volts to about 100 kilovolts. Variable inductance 46 is used formatching resonant conditions. Capacitance 50 alone represents cell 12when cell 12 is empty. When cell 12 generates a plasma, the power drawnby the generation of the plasma is represented by resistance 52.

The main limitation on the performance of power supply 24 is theperformance of switch 44. Solid state IGBT switches work well up tofrequencies of about 100 kilohertz at voltages up to one to twokilovolts. MOSFET switches can operate at frequencies up to several MHzat voltages between several hundred to several thousand volts, but asthe frequency is increased, the power that can be supplied by powersupply 24 with a MOSFET switch 44 decreases. In practice, the range offrequencies at which power supply 24 operates is from about 10 kilohertzto about 3 megahertz.

In operation, switch 44 is opened and closed at high frequency. Atypical mode of operation is opening and closing switch 44 at afrequency of one megahertz at 50% duty. When the switching frequency isequal to the resonant frequency of load capacitance 50 with theparasitic inductance of transformer 48 combined with variable inductance46, a high AC voltage is developed across capacitance 50. The maximumvoltage attainable is limited by circuit losses in power supply 24 andby power absorbed by resistance 52. The optimal voltage andinterelectrode gap width is a function of the pressure of gas stream 30.Device 10 may be operated at pressures of gas stream 30 from sub-Torrpressures to several Bars. Preferably, the pressure is on the order oftens of Torrs and the driving voltages are on the order of kilovolts.

Preferably, the high-frequency opening and closing of switch 44 isintermittent, a practice commonly known as “chopping”. This allows theplasma to relax and provides additional variation of the plasmachemistry. Preferably, this chopping is effected at a frequency betweenabout 10 hertz and about 100 kilohertz.

FIG. 5 is a schematic illustration of a preferred version of device 10,including mechanisms for adaptively controlling device 10 duringoperation. For clarity, DBD cells 12 a, 12 b and 12 c are represented asboxes, with the serial arrangement of cells 12 a, 12 b and 12 crepresented by output end 28 a being adjacent to input end 26 b andoutput end 28 b being adjacent to input end 26 c. Two kinds of sensorsare illustrated, one for measuring the concentrations of atomic, ionicand molecular species in gas stream 30 as gas stream 30 transits fromcell 12 a to cell 12 b and from cell 12 b to cell 12 c, and the otherfor monitoring plasma parameters such as temperature, electricalconductivity and plasma density within cells 12 a, 12 b and 12 c.

Gas species concentrations are measured by laser induced fluorescence.To this end, a collimated beam 62 of monochromatic light from a laser 60is directed by a beam splitter 64 into the region between output end 28a and input end 26 b and by a mirror 66 into the region between outputend 28 b and input end 26 c. Fluorescence excited in gas stream 30 bybeam 62 in the region between output end 28 a and input end 26 b and inthe region between output end 28 b and input end 26 c is detected byspectrometers 68 a and 68 b, respectively. This measurement arrangementis only illustrative. The scope of the present invention includes allsuitable apparati and methods for measuring gas species concentrations,for example by laser interferometry, by infrared absorptionspectrometry, or by simply diverting samples of gas stream 30 foron-line chemical analysis, for example using gas chromatography/massspectrometer or residual gas analysis. Plasma parameters are measuredusing Langmuir probes 70 a, 70 b and 70 c, which protrude into gaps 22a, 22 b and 22 c respectively via output ends 28 a, 28 b and 28 crespectively. Again, this method of measuring plasma parameters is onlyillustrative, the scope of the present invention including all suitableapparati and methods for measuring plasma parameters. Electrical signalsrepresentative of the readings obtained by spectrometers 68 a and 68 band Langmuir probes 70 a, 70 b and 70 c are conveyed by suitable inputlines 74 to a microcomputer-based control system 72.

Also shown in FIG. 5 is a source 80, of a pressurized additive gas suchas oxygen, nitrogen or hydrogen, connected to cell 12 c by anelectronically controlled valve 82 and a conduit 84. Plasma electrons inthe plasma of cell 12 c ionize the molecules of the additive gas tocreate free radicals and ionic species that react with the undesiredspecies of gas stream 30 and that interact with the original additivegas molecules. Conduit 84 is disposed to introduce the additive gas intogap 22 c of cell 12 c via input end 26 c. For clarity, only introductionof the additive gas into cell 12 c is illustrated. In fact, the additivegas may be introduced to all of the DBD cells of device 10. In addition,the additive gas may be introduced to gas stream 30 before gas stream 30enters device 10 or after gas stream 30 leaves device 10.

Control system 72 transmits control signals to power supplies 24 a, 24 band 24 c and to valve 82 via suitable control lines 76. The outputfrequencies and voltages of power supplies 24 a, 24 b and 24 c and therate of flow of the additive gas into cell 12 c thus are adjusted bycontrol system 72 in accordance with the readings obtained fromspectrometers 68 a and 68 b and from Langmuir probes 70 a, 70 b and 70 cto maximize the destruction of undesired gaseous species in gas flow 30.If one of the cells of device 10 is constructed in is the manner of cell12 h of FIG. 3, gap width 21 also can be adjusted, by appropriatesignals sent from control system 72 to stepping motors 38. For any givengaseous pollution abatement situation, it will be straightforward forone ordinarily skilled in the art to determine how to optimize thefrequencies, voltages, gaps and gas flow parameters and to programcontrol system 72 accordingly. For example, one optimal set ofparameters for the abatement of the fluorine-containing gases listedabove includes a pressure range for gas stream 30 is from about 0.1 Torrto about 200 Torr; a rate of flow on the order of a few hundred sccm forthe impurities in gas stream 30 and also on the order of a few hundredsccm for additive gases such as oxygen and hydrogen; and widths of gaps22 a, 22 b and 22 c between about 1 mm and about 4 mm.

Device 10 also can be operated at atmospheric pressure. This ability tooperate at atmospheric pressure greatly expands the range of situationsto which device 10 is applicable

FIG. 7 is a schematic illustration of an expanded embodiment 10′ ofdevice 10. In addition to DBD cells 12 a, 12 b and 12 c, device 10′includes three more DBD cells 12 j, 12 k and 12 l, also in series. Cells12 j, 12 k and 12 l are collectively in parallel with cells 12 a, 12 band 12 c. “In parallel” means, not that cells 12 j, 12 k and 12 l areelectrically in parallel with cells 12 a, 12 b and 12 c, for, indeed,like cells 12 a, 12 b and 12 c, each of cells 12 j, 12 k and 12 l hasits own high frequency power supply 24 j, 24 k and 24 l, respectively;but rather that one portion of gas stream 30 traverses cells 12 a, 12 band 12 c: entering cell 12 a via input end 26 a, exiting cell 12 a viaoutput end 28 b and immediately entering cell 12 b via input end 26 b,exiting cell 12 b via output end 28 b and immediately entering cell 12 cvia input end 26 c, and finally exiting cell 12 c via output end 28 c;and another portion of gas stream 30 traverses cells 12 j, 12 k and 12l: entering cell 12 j via input end 26 j, exiting cell 12 j via outputend 28 j and immediately entering cell 12 k via input end 26 k, exitingcell 12 k via output end 28 k and immediately entering cell 12 l viainput end 26 l, and finally exiting cell 12 l via output end 28 l.Embodiment 10′ has higher net throughput than embodiment 10, to handlehigh-volume gas streams 30.

While the invention has been described with respect to a limited numberof embodiments, it will be appreciated that many variations,modifications and other applications of the invention may be made.

What is claimed is:
 1. A device for destroying unwanted gaseous species in a gas stream, the device comprising: (a) a plurality of DBD cells in series, wherein each cell includes an input end adapted for admitting the gas stream into the cell and an output end adapted for discharging the gas stream from the cell; (b) a first cell and a second cell of the plurality of DBD cells, wherein the input end of the second cell is adapted for receiving the gas stream that is discharged from the output end of the first cell; (c) a first electrode and a second electrode for each of said DBD cells, wherein the first and second electrodes are adapted for a position substantially parallel to the gas stream; (d) a dielectric layer proximal at least a first of the first and second electrodes of each of said DBD cells wherein the dielectric layer is adapted for a position between the first of the first and second electrodes, and the gas stream; and (e) an independent power supply for each of said DBD cells for providing alternating current to the first and second electrodes of each DBD cell, wherein each of the plurality of cells is adapted for forming a plasma such that gaseous species are destroyed through plasma chemistry.
 2. The device of claim 1, wherein each of said power supplies provides alternating current to said each DBD cell at frequencies between about 10 kilohertz and about 3 megahertz.
 3. The device of claim 1, wherein each of said power supplies provides alternating current to said each DBD cell at a voltage of between about 300 volts and about 100 kilovolts.
 4. The device of claim 1 further comprising sensor disposed to measure a concentration of at least one gas species at the output end of at least the first of the plurality of cells.
 5. The device of claim 4, wherein said sensor includes a spectrometer.
 6. The device of claim 4, further comprising a control system, for adjusting at least one parameter of said power supply that provides alternating current to the second cell, in response to a gas species measurement by said sensor.
 7. The device of claim 1, further comprising a sensor, operationally associated with one of said DBD cells, for measuring at least one plasma parameter within said one of said DBD cells.
 8. The device of claim 7, wherein said sensor includes a Langmuir probe.
 9. The device of claim 7, further comprising a control system, for adjusting at least one parameter of said power supply that provides said alternating current to said one of said DBD cells, in response to the one plasma parameter measured by said sensor.
 10. The device of claim 1 further comprising a mechanism for introducing an additive gas into the gas stream at the input end of at least one of the plurality of DBD cells.
 11. The device of claim 1, wherein at least one of said DBD cells includes a mechanism for adjusting at least one parameter of an interior geometry of said at least one cell.
 12. The device of claim 11, wherein said at least one parameter of said interior geometry includes a gap width of said at least one cell.
 13. The device of claim 1, wherein at least one of said alternating current power supplies is a switching mode resonant power supply.
 14. The device of claim 13, wherein said at least one switching mode resonant power supply includes a switch selected from the group consisting of solid state IGBT switches and MOSFET switches.
 15. The device of claim 1, wherein at least one of said DBD cells includes an aperture for increasing a pressure of the gas stream within said at least one DBD cell.
 16. The device of claim 1, further comprising at least one DBD cell in parallel with said plurality of DBD cells.
 17. The device of claim 1, wherein at least one of said DBD cells includes an interior surface exposed to the gas stream, and wherein said interior surface includes a catalyst.
 18. The device of claim 17, wherein said catalyst is selected from the group consisting of black platinum and titanium.
 19. The device of claim 1 additionally comprising an adaptation of the first and second cells such that destruction of the gaseous species requires at least the first cell and the second cell.
 20. A method for destroying gaseous species in a gas stream in a device including a plurality of DBD cells in series, each cell having: (1) a first electrode, (2) a second electrode, (3) a dielectric layer proximal a first of the first and second electrodes, and positioned between the first and second electrodes and the gas stream, (4) input end for admitting the gas stream into the cell, (5) an output end for discharging the gas stream from the cell and (6) an independent power supply for providing alternating current to each of the cells, the method comprising: (a) activating the power supplies of at least a first and a second cell of the plurality of cells; (b) causing the gas stream to flow through the plurality of DBD cells, such that the gas stream flows (1) substantially parallel to the electrodes, (2) substantially parallel to the dielectric layer and (3) from the first cell to the second cell; (c) forming a plasma in at least the first cell; and (d) destroying gaseous species through plasma chemistry in the first cell.
 21. The method of claim 20, wherein said alternating currents are provided at frequencies between about 10 kilohertz and about 3 megahertz.
 22. The method of claim 20, wherein said alternating currents are provided at voltages between about 300 volts and about 100 kilovolts.
 23. The method of claim 20 further comprising detecting a concentration of at least one gas species emerging from at least the first DBD cell.
 24. The method of claim 23, further comprising adjusting at least one parameter of the alternating current provided to the first DBD cell in response to the detected concentration of said at least one gas species.
 25. The method of claim 20 further comprising measuring at least one plasma parameter within at least one of the plurality of DBD cells.
 26. The method of claim 25, further comprising adjusting at least one parameter of said alternating current provided to said at least one of said DBD cells, in response to said measurement of said at least one plasma parameter.
 27. The method of claim 20 further comprising chopping said alternating current.
 28. The method of claim 27, wherein said chopping is effected at a frequency between about 10 hertz and about 100 kilohertz.
 29. The method of claim 20, further comprising introducing an additive gas to the gas stream before the gas stream enters at least one of said DBD cells.
 30. The method of claim 29, wherein said additive gas is selected from the group consisting of oxygen, nitrogen and hydrogen.
 31. The method of claim 20, wherein at least one of said DBD cells has an interior geometry, the method further comprising adjusting at least one parameter of said interior geometry while the gas stream flows through said DBD cell.
 32. The method of claim 31, wherein said at least one parameter of said interior geometry includes a gap width.
 33. The method of claim 20 additionally comprising: (a) forming a plasma in the second cell; and (b) destroying gaseous species through plasma chemistry in the second cell.
 34. A device for destroying unwanted gaseous species in a gas stream, the device comprising: (a) a plurality of DBD cells in series; (b) for each of said DBD cells, a power supply for providing alternating current to said each DBD cell; and (c) at least one of said DBD cells including (1) an inner electrode having an axially varying transverse width and (2) an outer electrode surrounding at least a portion of the said inner electrode. 